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BLACKHOLE(4) BSD Kernel Interfaces Manual BLACKHOLE(4)
NAMEblackhole -- a sysctl(8) MIB for manipulating behaviour in respect of re-
fused SCTP, TCP, or UDP connection attempts
SYNOPSISsysctlnet.inet.sctp.blackhole[={0 | 1 | 2}]
sysctlnet.inet.tcp.blackhole[={0 | 1 | 2}]
sysctlnet.inet.udp.blackhole[={0 | 1}]
DESCRIPTION
The blackholesysctl(8) MIB is used to control system behaviour when con-
nection requests are received on SCTP, TCP, or UDP ports where there is
no socket listening.
The blackhole behaviour is useful to slow down an attacker who is port-
scanning a system in an attempt to detect vulnerable services. It might
also slow down an attempted denial of service attack.
SCTP
Setting the SCTP blackhole MIB to a numeric value of one will prevent
sending an ABORT packet in response to an incoming INIT. A MIB value of
two will do the same, but will also prevent sending an ABORT packet when
unexpected packets are received.
TCP
Normal behaviour, when a TCP SYN segment is received on a port where
there is no socket accepting connections, is for the system to return a
RST segment, and drop the connection. The connecting system will see
this as a "Connection refused". By setting the TCP blackhole MIB to a
numeric value of one, the incoming SYN segment is merely dropped, and no
RST is sent, making the system appear as a blackhole. By setting the MIB
value to two, any segment arriving on a closed port is dropped without
returning a RST. This provides some degree of protection against stealth
port scans.
UDP
Enabling blackhole behaviour turns off the sending of an ICMP port un-
reachable message in response to a UDP datagram which arrives on a port
where there is no socket listening. It must be noted that this behaviour
will prevent remote systems from running traceroute(8) to a system.
WARNING
The SCTP, TCP, and UDP blackhole features should not be regarded as a re-
placement for firewall solutions. Better security would consist of the
blackholesysctl(8) MIB used in conjunction with one of the available
firewall packages.
This mechanism is not a substitute for securing a system. It should be
used together with other security mechanisms.
SEE ALSOip(4), sctp(4), tcp(4), udp(4), ipf(8), ipfw(8), pfctl(8), sysctl(8)HISTORY
The TCP and UDP blackhole MIBs first appeared in FreeBSD 4.0.
The SCTP blackhole MIB first appeared in FreeBSD 9.1.
AUTHORS
Geoffrey M. Rehmet
BSD September 6, 2015 BSD